.login{
  position: fixed;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  background: url(../image/bei.jpg);
  background-repeat: no-repeat;
  background-size: 100% 100%;
}

form{
  width: 400px;
  height: 360px;
  border-radius: 50%;
  background: rgba(255,255,255,.5);
  padding-top: 40px;
  position: absolute;
  left: 50%;
  top: 50%;
  margin-top: -200px;
  margin-left: -200px;
  z-index: 2;
}
form::before{
  content: ' ';
  position: absolute;
  left: 50%;
  top: 50%;
  margin-top: -225px;
  margin-left: -225px;
  width: 450px;
  height: 450px;
  border-radius: 50%;
  background: rgba(255,255,255,.4);
  z-index: -1;
}
form::after{
  content: ' ';
  position: absolute;
  left: 50%;
  top: 50%;
  margin-top: -250px;
  margin-left: -250px;
  width: 500px;
  height: 500px;
  border-radius: 50%;
  background: rgba(255,255,255,.2);
  z-index: -1;
}
.logo{
  width: 100px;
  height: 100px;
  line-height: 100px;
  border-radius: 50%;
  text-align: center;
  font-size: 20px;
  margin: 0 auto;
  background: #fff;
  margin-bottom:12px;
}
.username,.password,.btn{
  width: 260px;
  margin: auto;
  margin-bottom: 20px;
  
}
input{
  width: 260px;
  height: 40px;
  padding-left: 12px;
  outline: none;
  border-radius: 4px;
  border: 1px solid #ddd;
}
.btn input{
  width: 272px;
  background: rgb(32, 129, 194);
  color: #fff;
  border: 0;
}